migration-planner

Use a dual-write migration pattern: write to both old and new databases, backfill historical data in batches, validate with shadow reads, then gradually shift read traffic using feature flags before decommissioning the old system.

Dual write migration writes data to both the old and new systems simultaneously while treating the old system as primary. After backfilling and validating the new system, you flip reads and eventually stop writing to the old system.

Validate with row count comparisons, random sampling of records (e.g., 1000 rows), and checksum comparisons across both databases. Run these checks continuously during dual-write and backfill phases, alerting if drift exceeds 0.1%.

Feature flags let you ramp read traffic to the new system gradually (1% to 10% to 50% to 100%) while monitoring errors and latency. If problems appear, flipping the flag off instantly rolls back reads to the old system.

Rollback becomes difficult after you stop writing to the old system, since new writes exist only in the new database. After decommissioning the old system entirely, rollback is no longer available, making it the point of no return.